1. Field of the Invention
This invention relates to a polyphenylene sulfide containing covalently bonded chlorine in an amount of 500 ppm or less, as well as to a process for producing said polyphenylene sulfide. Polyphenylene sulfide (hereinafter abbreviated to "PPS"), having excellent heat resistance and excellent chemical resistance, is widely used in electrical or electronic components, automotive or other mechanical parts, etc. Being moldable into a shaped article, a film, a sheet, a fiber, etc. by means of injection molding, extrusion molding, etc., the applications of PPS have been rapidly expanding in recent years.
2. Description of the Prior Art
Japanese Patent Publication No. 3368/1970 discloses a general process for producing a PPS, wherein an alkali metal sulfide is heated in an aprotic polar solvent to free water of crystallization from the alkali metal sulfide and then a dihalobenzene is added, followed by thermal polymerization. Japanese Patent Publication No. 12240/1977 discloses a process for producing a PPS of high polymerization degree, wherein an alkali metal salt of a carboxylic acid is added as a polymerization aid.
In these processes of PPS production, sodium chloride is formed as a by-product in an amount approximately the same as that of a desired polymer, namely, PPS. This makes it difficult to completely remove sodium chloride from the PPS with an ordinary treatment such as water washing. Moreover, the PPS contains chlorine and sodium bonded to its polymer chain terminals. As a result, commercially available PPS's contain about 1,000 to 3,000 ppm of sodium and about 2,000 to 4,000 ppm of chlorine. When a shaped article made from such a PPS is used in electrical or electronic components, the components have deteriorated electrical characteristics due to the large amounts of sodium and chlorine contained in the PPS and this causes big problem. For instance, when a PPS containing large amounts of sodium and chlorine is used for sealing of electronic components such as integrated circuits or the like, it is known that said sodium and chlorine reduce, due to their hydroscopy, the insulating properties of circuits or cause the corrosion of electrodes and lead frames, this leading to wire breakage, and thus characteristic deterioration and failure of the elements is induced.
In order to improve the above drawbacks, Japanese Patent Public Disclosure (Laid-Open Publication) No. 156342/1980 discloses a method wherein a PPS powder is subjected to extraction with hot water several times to reduce the content of water-extractable sodium to 100 ppm or lower. However, a study by the present inventors on this method revealed that repeated extraction over a very long time removed only sodium present on the surface or in the surface layer of PPS polymer particles and the polymer particles after extraction still contained 1,000 ppm or more of sodium. Thus, the efficiency of sodium removal by this method was low. Further, the PPS purified by the above method was measured for the content of covalently bonded chlorine present therein, and this indicated the presence of about 2,000 to 3,000 ppm of covalently bonded chlorine. Hence, no effect representative of the reduction of covalently bonded chlorine could be seen in the PPS.
Japanese Patent Public Disclosure (Laid-Open Publication) No. 219331/1984 discloses a method wherein a PPS is heat-treated in an aromatic solvent to reduce the sodium content. In this method, reduction of sodium content was possible but the PPS after heat treatment contained about 2,000 to 3,000 ppm of covalently bonded chlorine. Therefore, there was no effect representative of the reduction of covalently bonded chlorine.
As stated above, in the purification of PPS by conventional methods, reduction of sodium content is possible but the content of covalently bonded chlorine cannot be reduced. Therefore, PPS's made in conventional production processes have not been fully satisfactory for use as coating or encapsulating materials for electrical or electronic components.